Without realising it I seem to have become a Bottlehead fan, groupie, slave-to-the-tube... call it what you will. So after much discussion on the Crack-specific thread (http://www.head-fi.org/t/476650/crack-bottlehead-otl) I'm starting this thread for the discussion and comparison of all Bottlehead amps together.

The aim of this thread is to allow comparison discussions and to help people decide on the amp that best meets their skill level, listening preferences, headphones, and budget (but we all know that last one can change quickly once you're hooked).

Reviewing Specifics

Beyerdynamic T1 headphones only (I will comment on the pairing of the non-OTL amps with lower impedance cans, but direct comparisons will involve the T1 only to be fair and consistent)

PC audio delivered bit-perfect from Media Monkey via Chord SilverPlus USB to Audio-gd NFB-5.2 with a possible change to the Matrix X-Sabre soon. All comparisons will be completed with the same DAC meaning that I will go back and re-test with the X-Sabre if I upgrade. Edit: Upgraded to Matrix X-Sabre 20/10/13 - see product threads for notes on the impact of this change.

The same RCA cables between the DAC and amps - a set of high quality Stinger RCA cables that I favour due to their smooth, but detailed presentation

Standard, generic power cables (all reviews using 240V versions of the amplifiers)

Standard tubes as delivered with the kit (with a subsection in each review dedicated to tube rolling experiences)

The following review refers to the Bottlehead Crack OTL amplifier kit built in its stock form using only the parts supplied by Bottlehead. There are various modifications possible, most involving either different / extra capacitors or different volume pots and attenuators, but the sage advice offered by manyof the Bottlehead veterans on the Bottlehead forum is to build the kits in stock form before modifying. It's great advice because it gives you a baseline to work from if / when you decide to modify your kit.

The Crack costs around $300 depending on your local currency and shipping costs. You may also need to buy a few tools to build it (like a soldering iron / station and a test meter), but these are good to have once you start down the very addictive DIY path.

Build

Building the Crack is a straightforward affair even if you're an electronics novice. I had soldering experience prior to this project, but no circuit design experience. Although not strictly necessary, I'd recommend learning how to solder effectively before starting (there are some great videos on YouTube including this one from Inner Fidelity: http://www.youtube.com/watch?v=s5rWCEux_BI).

Parts & Assembly

You'll receive a box from Bottlehead which contains every piece you need to build your Crack. This includes a bag of resistors, capacitors and similar components, 4 pieces of pre-cut wood to build the box, 2 tubes, an aluminium chassis plate, and some other components like the volume knob, tube sockets and the like.

Instructions are provided on a CD as a PDF so you can print them off or view them on a computer / tablet. The instructions are widely acknowledge for their outstanding clarity and detail and are likely the reason that so many newbies can successfully complete the Crack as their first ever DIY project. It's not a difficult build, but the instructions are just perfect!

Stage 1 of the build requires some simple mounting of components using just a pair of long-nose pliers and a phillips-head screwdriver. Some of the components like the power inlet and power switch just push into place using a simple snap-lock system.

At the end of this stage, you'll have a chassis plate that looks something like the image to the left.

It's worth adding at this stage, that if you want to do any custom painting or finishes that they should definitely be completed prior to this stage. You don't want to be trying to paint components once they're assembled, but I'll speak more about this in the SEX review below.

You may also choose to assemble the wood base at this stage too. I just used PVA / hobby glue to build mine and then I stained and varnished the timber.

The final stage of assembly is the numbering system. The instructions guide you through a simple numbering system where you allocate numbers to each terminal by writing on the back of the chassis plate using a permanent marker. The result is an almost fool-proof system that lets you literally just connect #1 to #7 (or whatever the instructions say).

Hook-Up & Soldering

Stage 2 of the build is the hook-up and soldering process. These happen separately and simultaneously depending on the component. Sometimes the instructions tell you to connect a wire or component from A to B, but not to solder it yet. Other times, the instructions will say to connect and solder at the same time. This all depends on whether or not the terminals need to accept more than one connection (e.g. a wire and a capacitor lead) so be sure to follow the instructions carefully as it can be quite tricky to desolder some terminals.

An important note about soldering: make sure that each joint is well-soldered, clean (i.e. all excess wire trimmed off) and shiny before continuing on. It never ceases to amaze me the strange and profound effects that a poor solder joint can have on an entire circuit. Just because there's some solder on a terminal doesn't mean it's properly connected so take the time to double-check each joint before moving to the next stage. A good solder joint should be well covered in solder, but not dripping, and the solder should look shiny, not dull or cloudy. If in doubt you can always re-heat the joint until the existing solder flows again and add a little solder if necessary.

Testing

The final build stage is testing and it involves 2 (or 3) steps.

The first step is impedance testing where you leave the amp disconnected from power and test for the resistance between various terminals. Once again the instructions here are very straightforward so just step through them methodically and everything should be fine. I'll explain what to do with anomalies in a moment.

The second step is voltage testing and it's perhaps the most daunting step because you're connecting the amplifier to live and potentially dangerous electricity. Please be sure to carefully read the safety precautions in the manual before you continue here.

Just like the impedance testing, the voltage tests are a simple terminal-by-terminal process only this time it's with the circuit active. It's very normal to receive readings here that are 10-15% higher / lower than the ones in the manual so that's no problem, but if you start seeing no reading where you should or if the readings are way higher than they should be there are a couple of things to do:

Test again to be sure (and check your test meter's earth connection)

Check the solder joints leading to and from that terminal

Jump onto the Bottlehead Forum Crack thread and ask the awesome community for some advice

Once the testing is complete you can either plug in your favourite headphones or move to stage 3 of testing which is simply plugging in some cheap headphones that you wouldn't mind sacrificing (they must be in working order to start with) in order to test that sound comes out as expected.

If your circuit passed all the tests there aren't likely to be problems, but I always feel better testing with a $30 pair of headphones than a $500-1000 pair of headphones.

Design

The Crack is what's called an Output Transformerless (or OTL) amplifier meaning that it only uses tubes to driver the headphones. Other amplifiers use transformers to manage the current and voltage supplied to the headphones, but this brings higher cost (for good sound) and potentially some issues / design challenges. Being an OTL design makes the Crack both very affordable and very clean and smooth across all frequencies, but it does restrict its compatibility a little.

Headphone Compatibility

OTL amplifiers generally have higher output impedances than other designs and that means they are best suited to high impedance headphones. The Crack's output impedance is around 120ohms which is really high compared to other designs that aim for as low as <1ohm. Don't be turned off by that though, it just highlights the Crack's specialty - high-impedance cans.

The Crack is specifically designed to drive headphones with high impedance (e.g. 300ohms and up). Don't bother trying to get the best performance out of your 32ohm headphones with the Crack - it just won't happen. Even my 120ohm FA-011 LEs sound flabby and loose with the Crack because of the impedance mis-match, but connect a set of Sennheiser HD6X0s, HD800s, or the various 250ohm and above Beyerdynamics and prepare to smile!

Tubes

The Crack comes supplied with 2 tubes: a small 12AU7 tube (left image) which handles the input stage of the circuit and a larger 6080 tube (right image) that handles the power for the circuit. In my limited understanding, the 12AU7 does the work of amplifying the signal while the 6080 essentially passes on the amplified signal and manages the interactions between the amplifier and the headphones.

It's worth noting a couple of things about these tubes. Firstly, the 12AU7 is still in high demand and there are plenty of unused old ones floating around (known as new old stock or NOS) and there are also brand new ones being manufactured.

The 6080s on the other hand are no longer being manufactured, but there seem to still be plenty around and some of the variants available (e.g. 6AS7G and 5998) are both beautiful to behold and equally as beautiful to hear.

Sound

While writing this review I've been listening to my Beyer T1s (600ohm) via the DAC and amp in the Audio-gd NFB-5.2. I thought this provided a nice baseline given that the DAC stage of the NFB-5.2 will be driving the signal to the Crack for the next part of the review.

The following comments are based on the Crack with the Speedball add-on, but I will provide a full description of the difference with / without the add-on on a later section so don't worry if you just want to start with the Crack and not the Speedball, you can read all about the differences further down.

Treble

The treble from the Crack (stock tubes) is clean and detailed, but smooth. I don't feel like I lose any extension compared to the solid state NFB-5.2, but I seem to gain smoothness that makes the treble more enjoyable.

Tubes are known to introduce some slight distortion into the sound, but that distortion can actually be beneficial because it can smooth out the way we perceive the sound making it more musical and enjoyable.

In addition to the perceived smoothness, I also feel like the lower treble is a little more forward than with the NFB-5.2. The sound has some extra energy and life via the Crack, but without any harshness. The treble presentation is definitely more fun with the Crack, but still sounds accurate and true to life.

With the T1s being a headphone that can bite when fed poor quality treble I can definitely vouch for the Crack's treble being highly enjoyable and smooth.

Mids

Right on cue as I began to review the mids, Boz Scaggs' Sierra came on from my playlist. Sierra has some great vocals, congas, acoustic and electric guitars to really show off the mids and the Crack excels.

Mid-range presentation is smooth and clean with plenty of texture. It errs on the side of warmth and is a little creamy, but that's part of its seductive nature. Guitars sound full and realistic, vocals sound smooth and intimate, and yet the congas off to the right of the stage in Sierra sound light and lively.

On other tracks this trend continues. The Crack tames any dryness or edge in the mid-range of various recording and consistently delivers a smooth, creamy sound without getting bogged down in muddy or foggy presentation. I would say that it's not the most transparent presentation I've heard, but it walks the line beautifully between warmth and clarity - not too warm and not too stark. The Crack's presentation is about enjoyment, not dissection of the music.

Bass

The bass from the Crack is similar to the mid-range. It's solid and present, smooth and warm, but still with plenty of texture and detail.

The bass has a slight bloom to it that's not present from the solid state NFB-5.2 so it's possibly a touch slower than the actual recordings, but it doesn't muddy the bass so much as warm it up a little. Fast bass lines are still clean and defined and in some ways may be more true to a live / loudspeaker rendition where rooms create some bass reverb and resonance. Purists may feel like the Crack is colouring and / or slowing the bass down and technically that may be true, but to my ears it never once detracts from the listening experience and actually makes it a smooth and enjoyable ride.

Remembering that the output impedance of the Crack is 120ohm, the damping factor (ability to control the movement of the driver) between the Crack and the headphones will always be less than low output impedance amps so it's to be expected that the bass from the Crack will be a little fuller and warmer. Once again, it's not flabby or loose (so long as you're using high impedance cans), but full and a touch warmer than neutral.

Staging & Imaging

OK, this is the bit that really "wow"s me with the Crack. Moving from the NFB-5.2 to the Crack is like moving from the room next door to the band to actually standing right in front of the band. The staging and imaging from the Crack is SO far ahead of the NFB-5.2.

The stage on the Crack is still intimate, but it's open and spacious. Using the T1s, the stage extends just beyond each ear, a little bit forward, and from around eye-brow height down to the tip of my earlobes (I have average length earlobes ). So it's not huge, but it feels spacious.

Each instrument and sound is clearly placed in the stage by the Crack. I never feel like the instruments are crowded or their locations are smudged. To be honest, I didn't expect this kind of clear imaging from a tube amplifier. I don't know why, but I thought they'd be a little muddy and vague with their staging, but I can assure you the Crack isn't. It's precise, spacious and coherent across all instruments and recordings that I've tried.

I often use Good Excuse by the John Butler Trio as a test track for imaging because a good system will define the vertical staging of the recording. Ideally you should hear John's voice higher in the stage than his guitar. The Crack definitely clarifies the stage vertically, but not as much as other amps I've heard (like the Bottlehead SEX - teaser alert!). In short I'd say the Crack provides excellent horizontal staging and imaging, a good sense of depth and better than average vertical layering.

Sound Summary

To wrap it all up, the Crack's sound performance is way above it's pricetag. If you're looking for a smooth and enjoyable sound without sacrificing clarity, staging and detail, the Crack is a great choice (for high impedance cans). I'm yet to hear an amp <$1000 which I outright prefer to the Crack. I'm not suggesting for a second that there aren't amplifiers in that price range that do things better than the Crack, but there's not an amp I've heard that's made me think the Crack wasn't up to the task, especially given it's price.

Lately I've tested a few amps including the Matrix M-Stage, Aune S2 Panda Mk2, Woo Firefly, a Sherwood vintage receiver, and the Bottlehead SEX (I'm sure I've forgotten a couple too) and I can comfortably say that nothing in this mix made me question the Crack's value or performance. It's an added bonus that you get the satisfaction of building it, the opportunity to easily modify it and the option to customise its appearance.

For this review I wanted to highlight 2 pairings - a really affordable upgrade option and a top-notch upgrade option.

Affordable Rolling

My first rolling experience involved a budget of no more than about $30 per tube. At the time I was using HD650s as my headphone of choice and settled on a great, affordable combination. My input tube of choice was the RCA 12AU7 Cleartop and my power tube of choice was the Mullard 6080WA.

The addition of the Cleartop 12AU7 (AUD$30) brought a noticeable improvement to treble clarity and extension. The higher frequencies were brought in line with the lower treble frequencies making the Crack a little sharper and cleaner sounding. With the stock 6080 tubes I find the Cleartop to be a little dry and edgy at times, but that's why it's paired here with the Mullard 6080WA.

I simply love the Mullard 6080WA - for the AUD$25 I spent on it, it's a steal! The 6080WA is a great complement for the Cleartop because it softens the overall presentation slightly, but not in a muddy way. It's like the 6080WA picks up the musicians and just moves them back a few feet. The result is a really detailed, but gentle presentation with slightly more space. I really love the stage and image created by the 6080WA and would highly recommend this combination to any HD650 owners.

At the same time that I bought this pair, I also grabbed a Mullard ECC82 (12AU7 equivalent) for AUD$30. I had to mention it here because it's my favourite tube despite getting little use. The reason I love it so much is that it creates an almost holographic stage and image - easily the best imaging 12AU7 variant that I've heard. The reason it doesn't get used much is because it's very smooth and my favourite power tubes are also smooth - too much of a good thing, you know?

Aspirational Rolling

If you're looking for a real treat and you don't mind turning your budget DIY amp into a $600-700 kit, then I can highly recommend the GEC 6AS7G brown base tubes (there are straight and curved base versions - both are very similar with the curved base being slightly better).

I was fortunate to be able to buy a straight base version from Head-Fi user, W0lfdog (thanks again, W0lfdog!) and as you can see in the image to the right, I paired it with the RCA Cleartop 12AU7.

I've already explained what the Cleartop brings to the table so let's focus on the GEC 6AS7G. Imagine the ultimate combination of smoothness and detail and you'll be getting close.

The GEC presents the sound similarly to the Mullard 6080WA that I discussed above only it brings with it extra layers of detail and a bit more space and depth.

Because it is a warm sounding tube, I find I prefer it paired with a brighter input tube like the Cleartop or one of the stock 12AU7s as provided with the original kit. Paired with my beloved Mullard ECC82, the sound is great, but I always feel like I'm missing a little something in the treble. With the addition of the Cleartop, the brightness and slight edge of the Cleartop is a perfect foil for the smoothness of the GEC. Because both tubes are detailed in their own ways, the combination provides outstanding clarity, detail and precision, but absolutely never becomes dry or fatiguing. I know the cost of the tubes here can be nearly the same as the cost of the kit, but it really is a worthy expense.

Speedball Add-On

The final topic for discussion here is the Speedball add-on.

The Speedball add-on influences the power management of the Crack by replacing a few resistors with a more "intelligent" power supply control. The image to the left here is the pre-Speedball Crack. Those 2 white bars near the bottom are replaced by the Speedball in addition to 2 other brown coloured resistors which are to the sides of the white bar resistors down near the bottom of the picture.

The Speedball is also a kit design so you build it before installing it. It's a little trickier than the Crack, but not difficult overall. Installation involves the removal of the resistors mentioned above and then the mounting and wiring of the Speedball boards using some plastic spacers to hold the Speedball away from the other circuitry.

To my ears, the results of the Speedball modification are kind of similar to finding the perfect combination of high-end tubes. The sound becomes more detailed and better controlled while maintaining the beautiful smoothness of the stock Crack sound. Although there are still benefits to be had by rolling tubes after installing the Speedball, the Speedball actually reduces the impact of the tubes.

I should explain that last point further. If the stock Crack with stock tubes is a 5/10 in terms of the total potential of the Crack, then adding the right tubes can take it to an 8-9/10. Adding the Speedball takes the stock Crack with stock tubes to 8/10. Adding the right tubes to the Crack + Speedball takes the Crack to 10/10.

For around $150 (again depending on local currency and shipping costs), the Speedball is a great, predictable upgrade. Where tubes can be faulty, wear out, or not sound how you want them to, the Speedball is 100% predictable - it will always make a stock Crack sound tighter, punchier and cleaner. It also upgrades the maximum performance of your Crack so you can reach the absolute potential of amazing tubes like the GEC brown base above.

Also it's probably worth noting that it can be fun (and even recommended) to build the Crack without the Speedball first in order to practice your soldering and confirm that the circuit is correct and working. You also get to hear the stock Crack before upgrading so you can hear the differences made by the Speedball.

I lived with my Crack for around a year before going to the Speedball and I can assure you that it's an outstanding amp even without the upgrade.

Closing Statements

The Bottlehead Crack might just be the most fun you can have with $300, a pair of headphones and a soldering iron.

Having just built the Bottlehead SEX kit, I can confirm that the Crack is many times easier and therefore a more relaxing and fun process, especially for a beginner. That's not taking anything from the experience of building the SEX, but the Crack is just so straightforward that there's no stress in the process, just fun, learning, and satisfaction.

The results are far ahead of the price and it offers literally years of ongoing experimentation with the Speedball kit and various tube rolling options and that's not even mentioning some of the crazy modifications you can get into with capacitors and the like - just ask Franken-Crack (see image to the right and check this link) owner and Head-Fier, Amcananey!

Happy Cracking everyone and please feel free to comment and ask questions in this thread or the dedicated Crack thread.

DAC Upgrade Update - 27th October 2013

A recent upgrade to my DAC has brought some very interesting changes.

The new DAC is a Matrix X-Sabre based on the very detailed ES9018 Sabre DAC chip. The extra separation and clarity offered by this DAC (compared to the previous Audio-gd NFB-5.2) has brought the Crack (with Speedball) very close to the S.E.X. in terms of overall sound performance. It's so close now that I could actually be completely happy with either amp when driving the Beyerdynamic T1s. For lower impedance cans, the S.E.X. is still the only choice due to the Crack's high output impedance, but with high impedance Beyerdynamic or Sennheiser cans I would probably choose the Crack simply because it is cheaper and still excellent.

I think the key to this change is the signature of the DACs. The NFB-5.2 was slightly warmer than the X-Sabre. The X-Sabre is still musical, but a little closer to neutral than the NFB-5.2. The result is a cleaner sound from the Crack - still warm and rich, but not smoothed over at all. The S.E.X. can sound a little dry at times in comparison which makes the Crack a bit more seductive - it soothes the ears with detailed, mellow tunes while the S.E.X. presents oodles of details and clarity, but isn't always soothing.

Bottlehead S.E.X. [Review]

The following review refers to the Bottlehead S.E.X. kit amplifier which is the mid-level headphone amplifier in Bottlehead's range. The S.E.X. also happens to drive speakers which is a nice piece of flexibility, but I'll get to that later.

This review refers to the non-C4S modded version of the S.E.X. with a later post being dedicated to the C4S modification (a Bottlehead add-on kit similar to the Speedball for the Crack amp).

The S.E.X. is a step up in price from the Crack and sells for around $600 (incl. shipping) depending on your local currency and shipping costs. For the extra money it offers increased versatility over the Crack and maybe better sound, but I'll save those details for later

Build

Before I discuss the build stages it's worth noting that the S.E.X. is an incrementally harder kit to build than the Crack. The same techniques are used, but there are many more connections, many more wires, some slightly tricky components, and various wiring options. This means you need to keep your wits about you and allow plenty of time. I built the S.E.X. with 2 friends and fellow Head-Fiers, Gikigill and Billymav (1 kit each), and we completed the assembly stage in around 3 hours. It then took us from 11am until 2am to complete the hook-up and soldering stage (including a long lunch and moderate length dinner). That's 15 hours in total, but realistically we were on the tools for about 12-13 hours.

All up, I'd recommend allowing 2 relaxed days worth of work to complete the hook-up and soldering of a S.E.X kit, plus a few hours for assembly and an hour or so for testing (assuming no issues arise).

Parts & Assembly

The carton containing the S.E.X. kit is a heavy beast, and when you open it you quickly discover why. In addition to the aluminium chassis plate, the S.E.X. kit includes a power transformer, two output transformers, and two plate chokes that look a lot like the output transformers so be sure to note which is which (it's easy to tell by the fact that the plate chokes have wires already attached).

As per the Crack (and all Bottlehead kits as I understand it), instructions are provided on a CD so that you can print or view the manual electronically. Once again the instructions are exceptionally clear and well thought out.

Also just likethe Crack, the first step for the S.E.X. is to assemble the components such as the chassis plate, terminal strips, RCA terminals, and tube sockets (plus a few other parts). This is just a simple case of following the instructions and wielding a screw driver and pair of long nose pliers from time-to-time.

Once all of the assembly is complete, you'll have something that looks a little like the image to the left except yours will be a silver / aluminium colour, not browny bronze like mine, but I'll get to that.

Now, when you see the image to the left, you might be thinking "Hang on, he said assembly only so far, but I can see wires!" and you'd be right. If you want to stay solder free in stage 1 you could just mount the power transformer only, but if you're happy to include about 5 minutes of simple soldering then you can pre-wire and install the output transformers and plate chokes.

At whichever stage you choose to install the chokes, etc. you need to make a decision. Unless you've ordered the optional impedance switching kit (not reviewed here), the wiring of the output transformers will directly influence what the amplifier is optimised to drive. You can wire the S.E.X. to drive a 4, 8, 16, or 32 ohm load and that basically means you choose to wire it to suit speakers and headphones (4 / 8 ohm) or for more dedicated headphone use (16 / 32 ohm). I chose 8ohm wiring because I'd like to be able to use it with some bookshelf speakers I have, but my main priority is headphones. If using the S.E.X. with particularly power hungry headphones you would probably want to wire for 32ohms or buy the impedance switching kit that allows you to change impedance by simply turning the amp off, lifting the chassis plate and throwing a switch or two (as opposed to rewiring the output transformers which is what I would have to do).

Once again similar to the Crack, the final step of assembly is attacking your chassis plate with a permanent marker to number each terminal for the upcoming hook-up stage. This numbering system is truly a godsend and helps both with wiring and trouble-shooting if needed. For example, when something's wrong with the tube heater power supply it allows one of the gurus from the Bottlehead forum to tell you to check the C-strip (the terminals numbered C1-C5) rather than trying to explain it component-by-component.

Custom Appearance

When I built the Crack I was too excited to pause for any kind of custom painting or finish (except for staining the wooden base) so the S.E.X. gave me a chance to play.

If it's your first ever kit you may find summoning the patience difficult, but it can be very rewarding. I was fortunate that I had to wait while we found a date that worked for all 3 of us in the build group so I had idle time to play with the looks of my S.E.X.

I really wanted a vintage, art deco look for my S.E.X. so I ended up choosing an bronze colour anodised finish on the aluminium chassis plate (costing around $60) and I painted the visible metal parts on the plate chokes and transformer bell using simple, cheap spray paints from the local hardware store. All up, the customisation cost around $80-90 and a whole lot of patience and prep-work.

If you're painting the metal parts you really need to do the prep well as many of the parts will have resin on them as part of the manufacturing process. The resin doesn't play nicely with primer or paint so take the time to remove it thoroughly first time or you'll have to do what I did and strip back your first attempts (yes, that's multiple attempts - it took me more than one go to get this right) to start again from scratch.

In addition to anodising the chassis plate, other options are to polish the plate or paint it. It all depends on the type of look you're after so don't be afraid to get creative and you can check out the Gallery thread over on the Bottlehead forums for plenty of inspiration.

One final tip: if you choose to paint or anodise the base you'll need to make sure you connect the earth terminal to bare metal by sanding or filing the metal around the screw hole for the earth terminal.

Hook-Up & Soldering

There's not a lot to say here that I haven't already covered in the Crack review above. As already mentioned, the S.E.X. kit has more components and more wires so it is a trickier process than the Crack, but the techniques and steps are essentially the same.

One thing I will mention is that working with the Schottky rectifiers on the power supply is quite tricky because space is limited and the rectifiers have very thick leads which don't easily bend. The only real tip I have for this section is plenty of patience. There are 4 rectifiers to install and not a lot of space so it can get mighty frustrating and a break and a few deep breaths may well come in handy

That said, now is probably a good time to include some general tips which apply to both the Crack and the S.E.X., but which became increasingly important in the more complex S.E.X. kit where space is at a premium and your concentration may fade as the hours go by.

Top Building Tips

Check and double-check the orientation of all components that have specified directions / alignments. This includes the tube sockets, power switch, diodes, and capacitors. One error here can completely ruin your day and may destroy the component in the case of capacitors. Not only that, but de-soldering is a real pain in the proverbial, especially in cramped spaces with lots of components.

Measure wire lengths based on the path they will travel. We found a few times that the lengths specified in the manual were a little too generous or a little tight. This might be because we were working with the metric measurements (mm not inches) and a few of these had minor errors. My approach next time will be to check the origin and destination of each wire and trace its path (around obstacles and components) to get a tidier and easier solution.

Read the instructions carefully. All of the information and guidance is there, but it can be easy to miss a key detail so always read each instruction again before finalising your work.

I'm not going to discuss the testing stage here so please refer to the Crack review for details about testing.

Design

The S.E.X. is a single ended triode design which essentially uses one triode tube per channel in combination with an output transformer. SET designs are simple (i.e. not lots of components in the signal path) and clean, but not particularly efficient in the scheme of things. For our purposes though (driving headphones and sensitive speakers) they appear to be a good choice and knowing the amount of testing and planning conducted by the crew at Bottelehead, the SET design has no doubt been chosen for its combination of performance and affordability (with neither being compromised, just maximised).

Compatibility

By including an output transformer in the S.E.X., Bottlehead have given us an amp that can drive all range of headphones (including lower impedance models) as well as speakers. That's a pretty versatile offering and although I won't be using my S.E.X. for speakers, I love that it can drive them and a brief test proved that it drove my bookshelf speakers admirably (but not set up properly or for long enough that I'm able to effectively review the sound).

The S.E.X. comfortably drives headphones of all different impedances although I'd recommend drawing the line before you get to the sensitivity and impedance of IEMs. I tried my GR07 MkIIs and found the hum to be pretty bad (amp wired for 8ohms) so to me this is a headphone and speaker amp, not an IEM, headphone and speaker amp.

As for using it with various headphones, I've tried 600 ohm Beyerdynamic T1s, 120 ohm Fischer Audio FA-011 LEs, 75 ohm Ultrasone HFI-680s and some old, cheap 32 ohm AKG K-66s. In all cases, the S.E.X. drove the headphones comfortably and well, but I'll get to the sound in a few moments.

Tubes

The S.E.X. utilises the very utilitarian 6DN7 tube which is apparently an old TV tube and no longer made, but still fairly readily available as new old stock (NOS) it seems.

From everything I've read, there is no benefit to be had with tube rolling on the S.E.X. because all 6DN7s are essentially the same. I'm yet to test this myself, but the overwhelmingly consistent opinions on this topic make my wallet feel a lot more secure than the Crack with it's never-ending range of new, used and ultra-rare tube options.

Back to the 6DN7 tubes... There are some variants floating around where the base is much thinner. These are referred to as coin-base tubes and may look preferable depending on your tastes, but are apparently no different in sound.

Sound

So here we are at the part that really matters. I'll start by talking about the S.E.X.'s sound in general terms before comparing it directly with the Crack.

Remember that the following comments are based on the S.E.X. in 100% stock form and without the C4S add-on.

Before I get into the separate aspects of the sound, I wanted to mention that there is a faintly audible hum when you turn on the S.E.X. with any headphones connected. The hum isn't really an issue once the music starts, but I thought it was worth mentioning the hum.

The Bottlehead site states that wiring the S.E.X. in its 32 ohm configuration produces a particularly low noise floor so perhaps the hum I'm experiencing is a result of the 8 ohm configuration. I've PM'ed Doc about this and will update once I have confirmation. I might also re-wire to 32 ohms if that's the case.

One other aside before I break down the sound: when I first went to test the S.E.X. I plugged in my RWAK100 as a source and my old, cheap AKG K-66s as a test headphone. The K-66s were never a particularly inspiring listen (hence why they're my sacrificial test headphone), but I was absolutely gob-smacked when I heard them from the S.E.X. I have literally never heard such an amazing transformation caused by an amplifier.

Now, I'm not suggesting that the S.E.X. will magically transform your crappiest headphones into world-beaters. What the K-66 experience showed me was that the S.E.X. has a great presentation which will lift many headphones to a higher level than other similar-priced amps, and that it synergises really well with the K-66s for some unknown reason. My other headphones still sound clearly better, but where the gap from the NFB-5.2 is exponential between the K-66 and other headphones, the gap becomes much narrower with the S.E.X.

Treble

The treble from the S.E.X. is crisp and clean with nice bite. Through the T1s (which can be aggressive with the wrong treble signal), the S.E.X. presents outstanding detail and clarity in the upper registers, but without any fatiguing edge. Where the Crack seemed to offer a slight bump to the lower treble, the S.E.X. seems to be more balanced across the whole treble range. At first I felt it might have had a slight bump to the upper treble, but I think that might be more that I'm used to a little roll-off from the Crack.

The S.E.X. presents some really nice sparkle with the treble. Across a variety of tracks and with all different headphones, the treble presentation is consistently excellent. Somehow the S.E.X. manages to present the treble clarity of a solid state amp combined with the smoothness of a tube amp - very impressive. Cymbals and percussion have really nice energy with the S.E.X., but it's not overdone or in-your-face.

Mids

The mids from the S.E.X. are very similar to the treble - crisp and clean. There's a transparency from the S.E.X. that pairs beautifully with the T1s and I'd love to hear what it can do for headphones like the HD650 which are great, but can sound a little opaque at times.

Turning to Sierra by Boz Scaggs, the S.E.X. produces lovely, warm, liquid mids for the vocals and guitars. The presentation is clean and clear with no sense of slowness or blur. Boz's voice has texture and air while still being smooth and lush. Once again I'd describe the presentation of the S.E.X. as akin to the clarity of a solid state amp, but with the smoothness of a tube amp.

One thing I really like about the mid-range from the S.E.X. is the texture it grants to the instruments. I've been flat-out stopped in my tracks a few times while listening to music as I work. I listen to music via headphones for around 4-5 hours per day while I work so I'm used to hearing the detail and clarity that headphones can provide. Having used the T1s with the Crack for some time now, I thought I was hearing everything in my music, but the S.E.X. proves me wrong almost daily, not by showing me any extra details as such, but by revealing the subtle character of those details.

For example, the other day my music was playing on shuffle when a was stopped dead by a piano chord in a track I know really well, but I can't remember the name of as I write this. What stopped me wasn't the detail in the rendition, but the sense of realism conveyed by the texture and resonance I could hear in the piano. As a piano player myself, I find it rare to hear a system truly recreate the proper acoustic sound of a piano with all of its richness and subtlety. What I find amazing about the S.E.X. (especially as a $600 amp) is that it does this really well. I guess it comes back to the transparency I mentioned earlier - if it's in the recording and you have headphones to do it justice, the S.E.X. will let you hear it by simply amplifying the signal and getting out of the way.

Bass

If there's one weak spot for the S.E.X. it might be the bass, but not because it does anything wrong per se, just because it is very transparent and therefore may not give some people the "oomph" they're looking for.

I personally like the S.E.X.'s bass presentation because it's clean, tight, quick and punchy, but there's no denying that it doesn't have the body of the Crack (full comparison shortly). If I'm honest, I think that the S.E.X. is probably the more accurate representation of the recording, but some people might want a little more if they're looking for a warm, smooth, tubey sound.

On the plus side, the S.E.X.'s well-controlled and accurate bass is a great foundation for it's brilliant mid-range and treble presentation because there's no bleed between the bass and mids. The bass hits and rumbles when it needs to, but it never blurs the edges of other frequencies. For that reason, it's great with headphones that have a good bass presentation to begin with, but may not pair so well with headphones that people traditionally might use tube-warmth to "fix".

Staging & Imaging

Good golly does this thing image well!

I mentioned the K-66 experience at the beginning of the "Sound" section. It was staging and imaging that I heard with the K-66s that told me I was in for a treat. The S.E.X. presents a soundstage that is both wide and deep. With the T1s the soundstage from the S.E.X. extends beyond each ear by about an inch or two. It also has good forward depth, but I think the key difference I'm noticing is that it extends out diagonally better than anything else I've heard so far with the T1s.

What I mean by that is that the soundstage from the S.E.X. is more semi-circular than triangular. To the left and right of the centre there is equivalent depth to the centre and sides. Other amps I've tried, specifically the NFB-5.2 and Crack as my references from this review, seem to compress the stage at the 45o mark to each side before expanding again at the sides and centre so the result is a kind of triangular stage. The S.E.X. doesn't do this and it makes the stage feel massive and spacious even though it probably doesn't extend much further (if any) in the centre and to each side when compared to the Crack.

Sometimes a large stage can feel too big and the results seems somehow incoherent or unrealistic, but I don't find that at all with the S.E.X. Everything is perfectly placed with space around each sound, but without separating one instrument from the next. As you'll read in the comparison to the Crack. It does everything the Crack does well and a little bit more when it comes to staging and imaging.

Turning to Good Excuse as I did in the Crack review, the checked the S.E.X.'s ability to layer the stage vertically and was pleased to find that it manages the vertical equally as well as the horizontal. John's voice was clearly slightly higher than the cymbals behind him and higher still than the sound of his guitar. Bravo Bottlehead!

While completing this review I got a bit tired of the same songs over and over that I'd selected for these reviews so I switched to a different playlist. One of the tracks that followed was Clint Eastwood by Gorillaz. In the background of the track is a old piano that sounds like it's recorded from an adjacent room (not to be confused with the main synth which is piano-like, but not a piano). Listening on the S.E.X. actually made me double-check that the distant sounding piano wasn't a mobile phone ringing in the next room - it was eerily lifelike.

Sound Summary

To summarise the sound from the S.E.X. I'd have to say things like clean, transparent, accurate and detailed. It's definitely a smooth listen, but not at all warm like you might expect from a tube amp. It's not sterile, analytical or dry, but it also doesn't gloss over everything with a rose-coloured hue. It just finds a really nice balance that has instantly placed it as my favourite amplifier of all time (so far).

Comparison to the Crack

I'm basing this comparison on the stock Crack + Speedball with stock tubes. I will also briefly compare the Crack with the GEC 6AS7G brown base, but want to start with the 2 kits in essentially their baseline states.

Moving between amps, both using the same DAC, RCA cables and headphones (Beyerdynamic T1), the difference is stark, but not in the way you might expect. The S.E.X. doesn't wipe the floor with the Crack, they just kind of shake hands and walk comfortably in different directions.

Where the S.E.X. is clean, transparent and open, the Crack is warm, smooth and intimate. Both are enviably good so my aim here is more to help you choose which you might prefer.

Signatures

The Crack's signature is undoubtedly warm. The bass is more present, the mids are lush and creamy, and the treble is ever-so-slightly rolled off to a smooth, but detailed finish.

Returning to the S.E.X. I find a cooler presentation, but not cold. Both amps are probably on the warm side of neutral, but the S.E.X. is almost right on the line whereas the Crack is 2 steps into warm territory. Bass is reduced on the S.E.X., but still present. The nearest analogy I can think of for the bass is the HiFiMan "RE" range of earphones. I've owned the RE-0 and RE-272 earphones, both of which are beautifully transparent, but often left me wanting a little more in the bottom registers. Anyone who's heard these earphones probably knows what I mean and might be getting worried about where I'm going with this, but don't worry!! My point in raising the RE earphones is to clarify that the S.E.X. does not go as far away from the bass as something like the REs. Is it leaner than the Crack? Yes, without a doubt. Would I describe it as lean or analytical? No, not at all. Accurate, clean and quick, but not lean or analytical.

To use a really inaccurate, but illustrative method of description, I would describe the Crack's signature as sounding like a diagonal frequency response starting above neutral in the bass, staying much the same through the mids and trending downwards in the treble. The S.E.X. on the other hand starts basically neutral and continues flat across the board - end-to-end flatness. For some that will be desirable. For others, it may be a detractor.

Staging & Imaging

Once again, the two amps are different here, not better or worse.

The S.E.X. presents that beautiful, open and spacious stage I've described above while the Crack is much more intimate. Despite the Crack's intimacy, it doesn't feel crowded or congested, just intimate. I know many people who like one or the other so this is once again a question of taste.

Jumping back and forth, there is no way to choose a winner based on staging because it's a preferential choice. Imaging though is a little different. There is absolutely no doubt that the imaging from the S.E.X. is a step ahead of the Crack. It's not miles ahead, but definitely a good step ahead. Where the Crack has accurate imaging, the S.E.X. has crystal clear and almost holographic imaging. This might be in part due to the extra space available in the S.E.X.'s soundstage.

Resolution and Detail

I'm jumping back and forth between the two amplifiers as I write this and although I hadn't originally planned to include this heading it became abundantly obvious that it is necessary.

The reason this is so important is the difference in resolution and detail retrieved by each amp. The Crack is very good with detail and clarity, but the S.E.X. is on a whole other level. There are sounds I hear from the S.E.X. that I can't hear from the Crack even once I know they're there. I think this is in part due to the extra treble extension from the S.E.X., but it may also be a difference in the coupling of the headphones with the output tube versus an output transformer - I'll leave it to more qualified minds to answer that question though.

Reality Check

I'm calling this a reality check for 2 reasons. Firstly because I wanted to confirm if I can match the S.E.X.'s level of performance just by rolling tubes in the Crack and, secondly, because the tubes involved bring the total cost of the Crack to the same level as the S.E.X.

I began by installing just the Cleartop 12AU7 in the Crack to see what the affordable little marvel could do...

The sound was clearly better defined once the Cleartop was in the circuit, but was it as good? Nearly... very nearly. The Crack still didn't quite have the top-end extension of the S.E.X., but it was near enough that you might not miss it if you weren't directly A/B-ing the two amps. The Cleartop didn't bridge the gap in terms of imaging and the detail level was still a fraction behind the S.E.X., but it was definitely closer.

Next step was to pull out the big guns with the GEC 6AS7G and the results were actually very interesting. The GEC refined the Crack's sound a bit, but I don't feel like it pulled it any closer to the S.E.X. in the areas that it falls behind. What it did do though was take the Crack's mid-range to a new level and perhaps a level not achieved by the S.E.X. in terms of creamy, liquid goodness. I guess what I'm describing here is that it made the Crack better at everything it's already good at without really changing the overall presentation.

Simplified Comparison

We all have different tastes and preferences so here's a summary of where each amp excels (remembering that this comparison was completed with the Speedball installed on the Crack. I'm only including factors where there is a clear difference. If something's not there it probably means they're about the same, but feel free to post any specific comparison questions in the rest of this thread.

Bass quantity / body - Crack

Bass speed / refinement - S.E.X.

Lush mids - Crack (esp. with GEC tube)

Treble extension - S.E.X.

Detail retrieval - S.E.X.

Intimacy - Crack

Stage size - S.E.X.

Imaging clarity - S.E.X.

Closing Statements

It feels like there's a lot to summarise here so hopefully this crystalises it effectively. In my mind the S.E.X. clearly deserves to cost more than the Crack. Not taking into account the fact that the cost of materials is undoubtedly higher, the sound from the S.E.X. is more refined, more detailed and just plain impressive.

I still love the sound from the Crack and if I was told tomorrow that it was the only amp I could use for the rest of my life, I'd be completely OK with that. Since building the S.E.X., though, the Crack has had close to zero airtime because the S.E.X. is just too good.

I love the subtleties in music. I love to hear the little textures that the musicians and producers chose to include, but that we often don't hear during day-to-day listening. These subtleties help me to continue rediscovering my music collection and the S.E.X. shares new subtleties with me daily.

The Crack is an incredibly enjoyable amplifier and flat-out unbeatable for the price (when driving high impedance cans), but the S.E.X. is on another level overall.

If I had to recommend just one amp, I'd ask if you want warm, musical sound (Crack) or smooth, detailed sound (S.E.X.). Ideally though I'd suggest that you try both - start with the Crack as a practice run for the S.E.X.

Even if you don't spend a further $200+ on tubes, the Crack is an exceptional buy and one that still holds it's weight against its twice-the-price big brother.

Of course, if you want to drive speakers and headphones or if your headphone collection consists of <300ohm cans like HE-500s, AKG Q / K series headphones, or Audeze LCDs then go straight for the S.E.X. and love every second of it!

DAC Upgrade Update - 27th October 2013

A recent upgrade to my DAC has brought some very interesting changes.

The new DAC is a Matrix X-Sabre based on the very detailed ES9018 Sabre DAC chip. The extra separation and clarity offered by this DAC (compared to the previous Audio-gd NFB-5.2) has brought the Crack (with Speedball) very close to the S.E.X. in terms of overall sound performance. It's so close now that I could actually be completely happy with either amp when driving the Beyerdynamic T1s. For lower impedance cans, the S.E.X. is still the only choice due to the Crack's high output impedance, but with high impedance Beyerdynamic or Sennheiser cans I would probably choose the Crack simply because it is cheaper and still excellent.

I think the key to this change is the signature of the DACs. The NFB-5.2 was slightly warmer than the X-Sabre. The X-Sabre is still musical, but a little closer to neutral than the NFB-5.2. The result is a cleaner sound from the Crack - still warm and rich, but not smoothed over at all. The S.E.X. can sound a little dry at times in comparison which makes the Crack a bit more seductive - it soothes the ears with detailed, mellow tunes while the S.E.X. presents oodles of details and clarity, but isn't always soothing.

Attenuator & Capacitor Upgrade - May 2014

During May 2014 I began to modify my S.E.X. and Mainline amps beginning with the volume pot in the S.E.X. which I replaced with a stepped attenuator, assembled using high quality Dale Vishay resistors. I chose this particular attenuator because the Dale resistors are known to be very transparent and are actually the same as those used in the Mainline kit. By all accounts, you can achieve similar results with other options - the key is transparency.

Attenuator Installation

The first thing to note if you are changing your attenuator (or potentiometer) is that you might need to carve out a bit of extra space from the locking channel where the pin / tab of the attenuator sits into the chassis plate in order to prevent it spinning. Thankfully, the aluminium chassis plate is easy to work with using either a hand-file or a dremel. You can see in the image to the right that I lengthened and slightly enlarged the stock groove in the chassis plate. This isn't visible from the top once the volume knob is in place so don't stress if it's not the tidiest job ever (so long as you don't overshoot the width of the volume knob, but that would take some extreme carelessness).

Once the hole is ready, mounting the new attenuator is as easy as the stock one. Remove the supplied nut and washer(s), insert the spline through the hole, align the locking tab / pin and tighten the nut from the top side of the chassis. That's it for mounting - simple! The next step is wiring and that can change depending on the model of attenuator you choose so please refer to the provided instructions for your attenuator of choice and the instructions from the S.E.X. kit. If you get stuck, the wonderful people on the Bottlehead Forum can also usually help.

Do be aware at this point though that you may find the stock volume knob sits a little higher than it used to. It's not a major drama, but for the aesthetically inclined, you might need to modify the stock knob or track down a different one if you want the knob sitting flush against the chassis.

Attenuator Sound Improvements

You're not going to make a change like this unless there's a payoff so let's discuss what happens when you change the attenuator. The exact changes may vary depending the brand / model used, but if you go for something known for transparency (like I did) then you'll get a nice increase in the detail and clarity of your S.E.X. The signature of the amp doesn't change significantly other than perhaps a slight bit more brightness, but it's subtle enough that I'd say to expect no real change in signature and just an increase in detail retrieval and texturing of sounds.

As an upgrade, I would say it is absolutely worth it. Including the time taken to modify the hole in the chassis I would say this is a 1-2 hour project and delivers a sound upgrade second only to the C4S mod. You can complete this mod on a budget of about $50 USD, but can also spend a bit more for more exotic attenuators if you want to.

Capacitor Upgrades

Next on my list was a double-barrelled capacitor upgrade. I approached the upgrade in 2 stages at the excellent advice of either @JamieMcC or @mcandmar - I apologise for not remembering which one gave that specific advice, but they both deserve kudos for their input and support over the course of multiple mod discussions around capacitors and the like. The reason for the 2-stage approach was to isolate the impact of each pair of capacitors, but the problem was that the sound was still limited by other caps in the system so I am going to summarise the whole upgrade in one hit because I can't truly describe the full potential of the FT-3 teflons I installed first without having upgraded the other (output) caps which then brought their own influence to the sound. Hopefully that makes sense...

I started off installing some FT-3 0.1uF teflon capacitors (the same as the ones used in the Mainline upgrade below) and immediately noticed improvements even though they require significant burn-in. These caps replaced the pillow-shaped brown / orange capacitors that sit over the tube sockets. Early improvements I noticed were cleaner, clearer textures in the sound and better layering of instruments in the soundstage (i.e. different instruments were now more clearly defined 'in space' within the soundstage). As I said above though, the full impact of the teflon caps couldn't reveal itself because they were hidden behind the stock output caps (which are good, but able to be improved).

Stage 2 was the installation of output capacitors - the ones that connect the sound to your ears (kind of). I chose the Mundorf Supreme Silver / Gold / Oil caps for this purpose after doing a lot of reading about this particular model's excellent warmth and midrange. I decided that the Mainline should stay as neutral as possible while the S.E.X. would become my fun, organic, musical amp. Don't get me wrong though - this doesn't mean I was trying to turn the S.E.X. into some kind of thick, syrupy mid-monster. I wanted musical, enjoyable sound that was still nimble, agile, liquid and accurate. The good news is that the mods were a resounding success.

With the Mundorfs installed, the S.E.X. instantly took another small step in a very positive direction. All of the sound was warmer and fuller, but not at all heavy or thick. Music is still well-paced and lively, but with a little more note weight (not quite as good as the Mainline in this regard, but closer) and more present mids.

The staging from the S.E.X. is still excellent, but the Mundorfs added to what I was already hearing from the FT-3 teflons by helping to deliver greater fine detail and textural cues.

No doubt there are even more (subtle) changes that took place as a result of my upgrades, but the inability to truly A/B makes a direct and detailed comparison impossible. What I can tell you by way of a summary is that the S.E.X. post-upgrade does everything good that I wrote about above only now slightly better and with a dash more soul and warmth. I imagine you could get similar improvements with different cap combinations too and encourage you to give it a go. Changing these 2 caps on the S.E.X. is a relatively easy process and it's fun to hear what results.

Bottlehead S.E.X. C4S Upgrade

What is a C4S?

My understanding of the electronics theory is a bit limited so perhaps one of the more technically qualified members can add some extra detail, but, in layman's terms, the C4S is a circuit which applies extra load (impedance) to the tubes. The extra load helps the tubes to operate in a more stable range and also helps to prevent the influence of external noise sources.

The predicted result of the C4S upgrade is better resolution, quieter background and tighter bass.

Is it Hard to Build?

No. This is the easiest Bottlehead kit I've built so far. You do have to work on a circuit board, but it is with simple components and with nothing that poses major risk of damage or failure. You can see in the image that it has very few parts and the one part that can cause trouble (the 4 tiny LEDs) are safely hidden on the other side of the circuit board when you're doing the soldering of their leads.

This kit is so simple that I was able to successfully build it in front of the TV in about 30-40 minutes. I still checked and double-checked everything to ensure no errors (and recommend others to do the same), but it's just too easy and cheap to not do this upgrade!

The Sound

It's all well and good for me to sit here and say you should do this upgrade, but there's no point if it doesn't significantly improve the sound so let's chat about that...

If you've read my previous reviews and comparisons of the Crack and the S.E.X. you'll know that I prefer the S.E.X. over the Crack by just a small margin. The area that the Crack (with Speedball) held my attention was in the bass department. The Crack consistently delivers more impact and body in the bass compared to the standard S.E.X.

On one hand I was hoping that the Crack would still win the bass battle after the C4S upgrade to the S.E.X. because I don't want to make it a redundant item in my collection. On the other hand, the S.E.X. garners the majority of my listening time and therefore would be great if it could be the clear winner in every category.

Let's start with the Bass

As the one area that I felt the S.E.X. was a little lacking (only by a hair) I am pleased to say that the C4S completely rectifies my concerns. The bass on the S.E.X. is now sweet, solid and DEEP. To my ears, the S.E.X.'s ability to produce sub bass frequencies has significantly improved. The bass is tight and clean, but deep and powerful in a way I feel was missing before.

Everything Else

I know I normally break down the individual details (mids, highs, staging, etc.), but I'll just be saying the same thing over-and-over. Simply put, the C4S does everything that Bottlehead promise. The sound is cleaner and noticeably more resolved. To my ears it also seems smoother - more relaxing, but without losing any of the detail and transparency that I loved about the stock S.E.X.

As a result of these improvements there is now a better presentation of the micro details and texture in recordings, I am rediscovering my music yet again! In addition to the greater detail retrieval, the stage is now better defined, but probably no larger (it was already great so this is completely fine). Instruments jump out of recordings because they are so beautifully defined, but the music stays wonderfully coherent and together as a whole even when you can pick out each individual piece.

In short, my Beyerdynamic T1s have never sounded so good. I was up very late last night because I always wanted "just one more track".

S.E.X. + C4S vs Crack + Speedball

I'm sorry to say this, Crack addicts, but S.E.X. is better than Crack. It's a no contest.

The Crack is still an outstanding amplifier that punches massively above it's weight (or price), especially with the Speedball installed, but the S.E.X. is just on another level once the C4S is attached.

What the Crack does differently to the S.E.X. is a warmer, fuller mid-bass which can make the music a bit more mellow and smooth overall (similar to how the HD650s are very enjoyable for their warm, mid-bass and midrange presentation). The bass from the S.E.X. is leaner in the mid-bass area, but natural and solid. Where it excels, as I've already mentioned, is the sub-bass region. While the Crack may have this same sub-bass, it's harder to notice behind the mid-bass. The S.E.X. keeps everything so beautifully balanced that every single sound and frequency is right there for you to feel, hear and experience to the fullest.

Could I be happy with just a Crack + Speedball having heard the S.E.X. + C4S? Yes, Crack+ is still SO good that it can be an end-game for many people (so long as you have >300ohm cans). The S.E.X. is for those who want the versatility to drive lower impedance cans and / or speakers, and for those who want to take another step towards audio nirvana - I'm getting close now - I think I can see it from here!

The following review refers to the Bottlehead Mainline kit amplifier which is the top of the line headphone amplifier in Bottlehead's range. Unlike other products in the Bottlehead range, the Mainline doesn't have any "standard" upgrade options like the Crack's Speedball or the S.E.X.'s C4S kit so the review that follows is both the stock Mainline kit, but also the peak version of the Mainline without getting into aftermarket, non-standard modifications.

The Mainline kit costs USD $1199 before shipping and represents a significant price jump over the S.E.X. and Mainline kits. This review will help you decide if it's worth the extra clams...

I've tried to format the images in this post to be as unobtrusive in the layout as possible, but I wanted to include plenty of images as there aren't many out there to show all the "ins-and-outs" of the Mainline. Depending on the browser, window size and other factors, I seem to get different results so I hope the following review formats nicely on whichever device you're using and I apologise if the images cut up the text too much - please drop me a line if it makes it too hard to read and I'll see what can be done.

Build

As with all Bottlehead kits, the Mainline comes in pieces for you to assemble and solder yourself and that's what makes it such a great option for Head-Fi enthusiasts - you get to pay less (because it's a kit) and you get to build it yourself for the satisfaction and understanding that comes along with it.

Parts & Assembly

As you'd expect for the top of the line kit from Bottlehead, it's also the most intensive in terms of parts and overall size. The box isn't much bigger than the S.E.X. carton, but it's jam-packed with components including chassis plate, power transformer, output transformers, switches, sockets, capacitors, diodes, more resistors than you can poke a stick at, and a bunch of different hook-up cable including single core, 2 core with drain wire, and even some Cat-5 for good measure!

It was only when I unwrapped the parcels shown on the right that I realised how many pieces made up this beast. And, by the way, the Mainline kit doesn't come with a fiancee like the one in the picture - you'll have to get one of those elsewhere.

Custom Appearance

If you've read my S.E.X.review above, you'll know that I like to customise my Bottlehead kits. You don't have to do anything if you don't want to and you can just build it straight from the box, but the beauty with receiving a kit in pieces like this is you have the opportunity to individually treat / paint / modify the parts before assembly. Doc B has recommended always treating the bell end (the cover for the power transformer) to prevent superficial rust, but that's the only recommended pre-assembly work if you don't feel the need to customise.

My custom treatment saw me once again return to Jimmy at Riga Craft in Cheltenham in Melbourne. Jimmy did an amazing job on the chassis plate of the S.E.X. and I hoped he could repeat the performance on the Mainline plate.

This type of work is not what Riga normally do and the Mainline proved more challenging for some reason. Jimmy did great work and made multiple attempts to get the chassis plate just right, but eventually had to concede that the plate would never be 100% blemish free. He was extremely helpful and the final product is a reflection of the challenges of anodising an item like this, not a reflection on Jimmy's professionalism and effort. That said, the finished product was a beautiful looking blue anodised finish with the tiniest of blemishes in 2-3 spots. Only 1 blemish would be visible in the final, assembled amplifier (see image to the left) so I decided to go with a chrome pin-stripe to cover it and also add a little bit of detailing to the finished kit.

All-in-all I was really happy with the finished look of the pin-stripe and it covered the tiny blemish perfectly so if you're thinking of anodising or painting your chassis plate, be prepared to improvise and get creative with things like pin-stripes because there is always a chance of minor blemishes in unfortunate locations no matter how careful you are.

Other than the chassis plate, I kept the rest of the customisation pretty simple with aluminium coloured engine enamel on the bell end and valve covers and a simple, linseed oil treatment on the wood base. I won't post images here because you'll get a chance to see it later in this review.

Hook-Up & Soldering

Much like the other Bottlehead kits, the Mainline doesn't require an engineering degree to build and is just a matter of following the instructions carefully and taking your time.

Stage 1 is the attachment of all the components to the chassis plate. This stage is very straight-forward and possibly easier than the S.E.X. because you don't have the top-and-bottom mounting of the chokes and transformers like you do on the S.E.X. You need to pay attention to the mounting of the tube sockets and switches to ensure the correct orientations, and there's a little bit of adjustment to the gain switches before you install them, but that's about as tricky as it gets.

Once assembly is done, the soldering of wires is much the same as the S.E.X. Bottlehead provide multi-core cables for the signal and power so there are less individual strands going all over the place. This keeps the process nice and neat, but means stripping and prepping multi-core cables which can be a little fiddly so there's a trade-off. For what it's worth, I like the end result with the neatness of the Cat-5 and 2-core cables so it's worth the minor hassle of stripping multiple layers of insulation.

Another part of assembly is the "building" of the attenuators. At the bottom of the image of the assembled, but un-wired chassis you will see two blue circles with multiple silver terminals. These are the gain switches and require the installation of a series of resistors between the switch terminals and the terminal strips on either side.

I was initially intimidated by this step, but can report that it's quite manageable (with care and attention) thanks to the always excellent instructions provided by Bottlehead. I managed to goof on one of the solder joints here which caused a channel imbalance on one particular gain setting, but after wasting some people's time over on the Bottlehead Forum (thanks guys and sorry!) I discovered a solder joint had formed perfectly around one resistor's lead while leaving a void around the other resistor connected to the same terminal. 2-3 seconds of heat from the iron and it was right as rain!

Circuit Boards

The Mainline is unique in the Bottlehead headphone amp range because it is very tightly regulated. The signal tubes are each provided with a C4S board (similar to the S.E.X. C4S upgrade), the power supply has its own circuit board featuring an array of capacitors and diodes (see image to the right), and there is a board with some trim pots to ensure that the power delivered to signal tubes is perfectly matched.

At first, I expected the circuit boards to make the Mainline more challenging to build, but in many ways it actually made the process easier because some of the tricky connections from the S.E.X. kit (e.g. the diodes on the power transformer) are addressed with much simpler circuit board assembly and then fly leads from the circuit boards to the relevant terminals on the tube sockets or transformer.

I did make one crucial error at this stage though...

When wiring the blue trim pots on the regulator board (bottom left of image to the right), I mis-understood the instructions and failed to look at the provided picture in the manual. This resulted in one terminal on each side of the circuit board being left out of the circuit and was a complete pain in the **** to fix. As always, the key with these kits is patience, care and double-checking before soldering. I failed on this occasion, but was thankfully able to fix the problem without any broken parts.

Top Building Tips

Much like the tips for the S.E.X. build, the Mainline is mostly about taking your time, being careful and double-checking the instructions before finalising a solder joint. Cable paths are much more defined on the Mainline so you don't need to think ahead about where to run each cable - Bottlehead have already done that for you and even provide cable tie-down points as part of the kit and instructions.

One tip that is a bit unique here though is to do your numbering of the terminals with a fine-tip marker. The marker I usually use is a bit thick and I had trouble marking some of the tighter spots (such as the tube socket terminals) and this came back to bite me later when I soldered a capacitor to the wrong terminals and had to re-do the connections.

Design

The Mainline employs a design with output transformers so it will pair with a wider range of headphones than something like the Crack (which is output transformer-less or OTL). It also has balanced and single-ended outputs which differentiates it from the S.E.X. which is single-ended only. To achieve the balanced output, the Mainline has a full-sized Neutrik 4-pin XLR socket in addition to a 6mm Neutrik locking headphone jack.

It's worth noting here that there is a switch to select between balanced and unbalanced operation and although sound comes through both outputs in either mode, the sound definitely changes when when switched correctly for the socket and 'phones you're using.

Another design element unique to the Mainline (in relation to the Crack and S.E.X.) is its use of attenuation switches rather than a volume knob. Front and slightly left-of-centre on the amp are 2 black knobs labelled "coarse" and "fine". To set the volume on the Mainline, you first adjust the coarse attenuation which jumps in 9dB steps (from 0dB to -45dB) and then tweak the exact volume using the fine adjustment which moves in 1.5dB steps (from 0dB to -7.5dB).

I don't think this approach is necessarily better in terms of usability compared to the simple volume pot, but it's been implemented for the quality of the signal path and it works really nicely. Of course there's no way I can compare the quality of the sound with this setup versus a traditional potentiometer so we'll have to take Doc's word for it, but I'm yet to be disappointed with any of Bottlehead's design choices so I think it's a safe bet that this is the best option they could create.

Compatibility

The Mainline has 2 different output impedances to suit high and low impedance headphones. There is a switch to select the mode you require and throwing this switch sends the signal through a different winding on the output transformer to provide more grunt to high impedance headphones, but at the cost of a higher output impedance. This makes the high output mode perfect for 300+ ohm Sennheisers and Beyerdynamics, but I'd stick to the low impedance mode for pretty much everything else just to ensure good damping and frequency response.

Listening with the Beyer T1s I definitely notice a slightly looser bass response on the high impedance setting, but it brings a desirable warmth and fullness rather than getting sloppy and loose. Flicking back to low impedance tightens up the bass, but sounds leaner as a result. Both settings sound excellent on the 600 ohm Beyers, but not so much on my 120 ohm Fischer FA-011 LEs, hence my recommendation of saving the high impedance mode for 300+ ohm headphones.

So far I've tried the Mainline with everything from 600 ohm Beyer T1s right down to 15.9 ohm Unique Melody Miracles. Normally, tube amps and headphone amps aren't great with sensitive IEMs, but the Mainline performed far better than I expected with only the faintest hum. The sound was excellent, but I think the output impedance is a little too high even on the low impedance setting to make the Mainline a good match with 16 ohm headphones or earphones and that's completely fine with me because this is a headphone amplifier designed for high-end headphones, not hyper-sensitive IEMs or low impedance headphones designed for portable use. It's got plenty of power for everything I've tried it with and it pairs extremely well with all sorts of headphones without any noise problems or impedance mis-matches.

Balanced Output

Having a balanced output means the Mainline can play nicely with balanced headphones (i.e. headphones provided with balanced connections or with aftermarket cables / modification). Some common headphones that can be run balanced are Sennheisers and Beyerdynamics with dual cable entry (cable goes to both cups) and aftermarket cables / modifications, as well as the HiFiMan HE series and the Audeze LCD headphones.

Switching between balanced and unbalanced on the Mainline makes only a subtle difference to the sound with I think a slight (~1dB) volume increase running balanced and perhaps a slight increase in resolution and control. It's not a dramatic change and I wouldn't recommend spending a fortune for balanced headphones to go with the Mainline, but it's definitely a nice addition if you already have balanced headphones and I love being able to have 2 sets of headphones connected - Beyer T1s and Fischer FA-011 LEs for now.

There was a bit of discussion in this thread about the fact that the Mainline has a balanced output, but only unbalanced inputs. According to the discussions, this decision was made based on experiences that a good unbalanced input is as good (or maybe even better) than balanced inputs due to the extra complications of input transformers for balanced inputs. Unfortunately there's no way for me to confirm that because there's no fully balanced Mainline that I can compare directly with, but given Doc's track record (see reviews above) I think it's safe to assume that it's a sound decision (pun slightly intended).

Tubes

The Mainline has 3 tubes in total. A 12AU7 is used to control the power while two 6C45-Pi tubes take care of the signal. From what I can tell there are really no alternatives to the 6C45 tubes and there's no point rolling the 12AU7 power tube because it doesn't influence the sound (I've tried).

This means that the Mainline, like the S.E.X. is a great amp if you don't want to fiddle around with rolling tubes, but may not be for you if you crave the opportunity to play with different tubes for their different sound characteristics. The Crack is the best (i.e. only) option for tube rolling in the Bottlehead range.

Sound

The part that really matters...

I said after installing the C4S in the S.E.X. that I was worried how good the Mainline might be if it was any kind of significant upgrade over the S.E.X. + C4S combo which is outstanding. The good news is that the Mainline didn't blow my mind as I feared it might after hearing the S.E.X. + C4S. The bad news is that the leap from the S.E.X. to the Mainline isn't as dramatic as I thought it might be, but don't be discouraged - it's worth reading more to fully understand what differentiates the Mainline from it's cheaper brethren.

Treble

Clean, crisp, extended and detailed, but oh so polite.

The Mainline delivers smooth, but fully extended treble without a hint of hash or grain. There's no lack of detail or clarity, but a complete lack of harshness and fatigue. The Mainline's treble presentation is the best I've heard as yet in my headphone journey. Cymbals, percussion and high frequency textures are present and clean with good weight and presence, but are well-balanced with the rest of the sonic picture. Music can still be sibilant, but only as a result of a poor recording, not because of the amplifier. As strange as it sounds, that's a good thing because it means the amp isn't adding harshness or edge to good recordings, but while also not smoothing over poor recordings - it's presenting what's there in a really polite, but accurate way.

Mids

The mids from the Mainline are simply sublime. There's a weight and substance to the mids that is reminiscent of the Crack at its best, but significantly better.

Of all the Mainline's sonic traits, the midrange is probably the defining factor to my ears and it's the weight of notes which signifies the difference. Instruments and vocals sound and feel real. I had never really noticed before that other amps of mine create a really enjoyable facsimile of the instruments in the recording. The reason I had never noticed is because I hadn't heard it done just right. Hearing the Mainline has redefined my expectation of midrange presentation from an amplifier. There's no thickness or creaminess to the midrange sounds, just a solidity and presence that's beguiling and magical.

I wondered for a while if I'd hit a sweet spot on certain tracks, but listening to a range of tracks showed that the Mainline excels on a range of instruments, male and female vocals, transient sounds (such as drum hits) and sustained notes - everything just sounds so real.

Bass

I've stated previously that the one short-coming of the S.E.X. in my eyes was it's slightly lean bass presentation. I'm pleased to report that the Mainline strikes a perfect balance between the Crack and the S.E.X. It's tighter than the Crack, but fuller than the S.E.X.

Similar to the mids, the bass from the Mainline is weighty and realistic. There is plenty of presence at the bottom end, but no bloat or bleed from the bass. As I mentioned earlier, the impedance switch can tighten or loosen the bass presentation slightly depending on your headphones, but on either setting (given appropriate impedance headphones) the bass is excellent - full, punchy, and quick with great impact, incredibly realistic weight, and just the right speed of decay.

There's not a lot more to say about the bass of the Mainline - it just does what it should - presents the bass in a realistic way that's accurate to the recording. There no emphasis or alteration that I can hear - just clean, accurate bass that's perfectly balanced with the rest of the audio spectrum.

Staging & Imaging

If I had to choose one area that the Mainline has failed to meet my expectations, it's in the area of stage size, but this needs some explanation...

The Mainline's stage is more intimate than the S.E.X.'s and I simply adore the space and size you hear when plugged into the S.E.X. The Mainline's presentation is more intimate than the S.E.X. - once again about halfway between the presentation of the Crack and the S.E.X. If the Crack is a front row seat and the S.E.X. is a 10th row seat, then the Mainline is probably a 3rd or 4th row seat. I'll discuss the differences between the Mainline and the S.E.X. in the comparison section.

The Mainline's staging is open and defined despite the more intimate placement of the music so it's not like the staging and imaging is poor, it's just not in line with my personal preferences. That said, I'm not sure that the incredible realism and weight I discussed earlier would be possible with the more distant presentation of something like the S.E.X. and I would trade that level of realism for anything

in the world.

You might be worried that a more intimate presentation equates to a congested or crowded sound - well don't be! Sounds are clearly separated and defined and the weight of notes discussed earlier make the imaging some of the most accurate and realistic I've heard. While only moderate in forward projection of the sound, the Mainline's stage is wide and has good height.

Perhaps the strongest aspect of the Mainline's staging and imaging is its ability to separate different sounds while maintaining a single overall auditory picture. This amp manages to keep a perfectly coherent representation of the musicians while also perfectly defining each sound in its own space. You can hear every instrument, vocal, noise and texture on it's own and yet also simultaneously as a part of the overall sonic tapestry. Sounds pop out of the recording when they're meant to and blend in when they're meant to, but at any time you can single out any individual sound or instrument and hear it completely on it's own without negative influence from other sounds in the recording. This all makes for an incredibly engaging listening experience and it's got me really wanting to try the Mainline with some HD800s to see what it's really capable of when it comes to staging and imaging.

Sound Summary

The Mainline's sound is effortless. It's controlled, polite and completely coherent. At first listen you may not be "wowed" by this amp, but on extended listening you soon realise that every track you hear sounds as solid and real as you've ever heard it. Nothing jumps out to amaze you, but everything sounds excellent.

I used to work for Bang & Olufsen and one of the key traits with their audio and video gear was an attention to lifelike reproduction. At first, lifelike reproduction sounds ordinary and everyday - it's only as you spend more and more time with it that you appreciation of it grows because it's completely believable and never fatiguing because nothing is overdone or enhanced. The Mainline reminds me a lot of this approach to audio - nothing is emphasised or highlighted so it's not an exciting sound, but it's a real sound and that continues to be enjoyable for hours on end and days, months and years of listening.

The sound of the Mainline is accurate, clean, smooth and controlled. It's incredibly balanced across all frequencies and it delivers the same auditory experience on every track I've tried across all kinds of genres. This is an amp you buy to listen to and enjoy for the long haul, not to "wow" you or amaze you in the short term. The realism of sound from this amp is still enlightening me as I continue to listen - it's a subtle kind of marvelous that has to be lived with for a while to fully understand, but it's addictive and amazing once it starts to sink in.

Comparisons to Crack (w Speedball) and S.E.X. (w C4S)

I've compared the Crack and S.E.X. comprehensively above, so I'm going to keep this fairly simple with descriptive explanations of how the 3 amps compare on certain traits. If you have any specific questions I'll be happy to respond in the thread.

Signatures

The Mainline's signature is most like the Crack w Speedball. It's smoother than the S.E.X. with a little less treble energy, but no less detail and the Mainline's bass is a notch or two stronger than the S.E.X. Flicking between the S.E.X. and the Mainline, I'd say the the Mainline has flatter bass response while the S.E.X. sounds like it has a slight mid-bass hump before rolling off in the lower registers. This is all really subtle stuff because they all sound great, but the Mainline is clearly the most neutral / balanced sounding of all the range across all frequency ranges.

Staging & Imaging

As I've already mentioned, the S.E.X. presents a wider, more spacious soundstage that I just love! The Mainline's staging is much more similar to the Crack - intimate but with more space around the sounds. For stage size alone, the S.E.X. actually wins this battle for me, but others might prefer the more intimate presentation of the Mainline.

In terms of imaging, I'd say the Mainline is easily the most realistic amp I've ever heard because of the weight of notes - everything sounds so real and present that it's a revelation. The S.E.X. is excellent in terms of imaging, but sounds like a excellent reproduction while the Mainline sounds like the real thing. The Crack is commendable for its price tag (running with the stock tubes), but can't compete with the other two in this area.

Resolution and Detail

I was expecting this to be really close, but it wasn't... The S.E.X. produces excellent details, but the Mainline just flat-out outperforms it from a couple of perspectives. Firstly, the smoother presentation means that nothing is overshadowed. Listening to the S.E.X. I occasionally lost details behind a little bit of excess treble energy, but the Mainline never does this - everything is beautifully balanced so you can hear everything in the recording. In addition to the smooth presentation, the weight of notes from the Mainline means that even subtle details are given presence in the recording. They don't over-step their place in the sonic landscape, but they are 100% there and you hear them all exactly how they were intended.

Reality Check

Just like I did for the S.E.X. / Crack comparison, I upgraded the tubes in the Crack to see how close I could get it to the Mainline. In its stock form, the Crack can't compete with the Mainline (and nor should it at 30% of the price), but how about with a $20-30 12AU7 and a $150 6AS7G (brown base GEC)?

Good news in all directions! The Mainline remains clearly better in terms of refinement, space between sounds, overall stage size, and resolution. However, the $600-700 version of the Crack offers astounding performance for those who prefer a more intimate presentation than the S.E.X., but don't want to go so far as the Mainline in terms of price.

The Crack remains an astounding achievement in terms of price-to-performance ratio and it's the only Bottlehead amp to provide the fun option of tube rolling to tune the sound.

Overall Summary of the Mainline

Overall, the Mainline clearly holds its place as the flagship Bottlehead amp and is currently the best amplifier I have ever heard. The fact that you can build it yourself, customise its appearance, run balanced and unbalanced headphones, select between 2 inputs on the fly, and adjust output impedance to suit different cans makes this an outstanding amplifier for those looking for an end-game flagship amp.

On first listen I was worried that the Mainline wasn't worth its price tag, but its real value is in how transparent and subtle it is - don't buy the Mainline to wow your friends with a short listen, buy the Mainline to amaze yourself for months upon days upon hours of listening.

Cap Upgrade Update

Over the past few weeks (April-May 2014) I've installed two sets of capacitors into the Mainline starting with a set of 18uF Auricap High Resolution capacitors in the position of the stock Dayton 10uF capacitors and a set of 0.1uF FT-3 teflon capacitors fresh from Romania replacing the stock 0.1uF Daytons on the Bias Regulator board.

FT-3 Teflons - 0.1uF 600V

I'm going to start this update with the teflons on the regulator board. I've literally just finished this installation and it may require some time for burn-in of the teflon caps, but I honestly can't hear any noticeable difference in the sound and would question the value given that the installation is a bit ugly and clunky due to the size of these capacitors. As you can see in the image to the right, I've had to run fly leads from the teflon caps which are cable tied to the Auricaps. I've also use some heat shrink tape to create an insulating buffer to keep the teflon caps sitting a little way off the Auricaps (even though there should be no issue with them touching due to the plastic coating on the Auricaps.

These FT-3s cost around $10 per pair plus postage (via eBay) so it's a small enough investment that I really don't mind if they have done absolutely nothing to the sound because they haven't taken anything away and if they provide improvements after burn-in then that's great.

I found some variation in the bias settings (adjustments to ensure identical voltages for each output tube) following the install so if you do change the regulator caps be sure to double-check the biasing if you want to maximise performance.

Auricap High Resolution Polypropylene - 18uF 400V

These have been in place for a few weeks now and are definitely a worthy upgrade. They cost me about $130 for the pair from a local supplier and are the best $130 I've spent in a while. As you can see in the pictures, they just fit in the space between the C4S boards and the output transformers, but still need a tie down using an adhesive anchor on the chassis (the same as the ones supplied with the Mainline kit to tie down the Cat 6 cables).

If you've read the review of the Mainline above, you'll know that the treble on the Mainline in stock form is extended and detailed, but still smooth and non-fatiguing. I did say in the original review that there's no hint of fatigue when listening to the Mainline, but I have to call myself a liar now and say that there must have been the tiniest hint of harshness / graininess there. I know that because now it's gone and its absence has left all of the Mainlines magic on full display. Don't get me wrong, the stock Mainline kit is in no way flawed. In fact, as I see it, the ability to spend $130 and 15 minutes to bring the amp to another level shows just how good this circuit is.

The sound with the Auricaps is basically identical to the overall sound of the stock Mainline only smoother, more defined and just better. Treble is smoother while still fully extended and detailed. Midrange textures are sophisticated and intricate, and the bass is extended, textured and punchy. As I said above, the best thing about the Auricaps in the Mainline is that they don't change a great thing. They keep the Mainline's signature and neutrality completely intact and just let it do what it does better. It's hard to say exactly how much influence the Auricaps have had on this next part, but I would swear that the Mainline now has a clearer, stronger image than before. This is most likely due to the slightly smoother treble allowing all the other auditory cues to arrive cleanly and accurately, but the staging and imaging now with the Beyer T1s is really magical. It was good before - outstanding even, but it definitely seems even better to these ears now. I am having "wow" moments multiple times each day.

One other note as to why I chose the 18uF rather than a direct replacement of the 10uF stock caps. From the reading I did, a larger capacitance can provide better bass response which I was keen to explore to see if there was any room for improvement in the case of the Mainline. The results are that if there's an improvement there I can't tell. The hardest challenge here is that you can't quickly A/B a capacitor change like this so it's all based on auditory memory and the difference in bass extension (if there is one) is small enough to be insignificant, unlike the change in sophistication and smoothness in the mids and treble.

Capacitor Wrap

The only other caps in the circuit that could potentially benefit from upgrading are the electrolytics used on the power supply board, but they fit so neatly on the board and are already good quality Panasonic capacitors so the potential benefits here seem questionable (although I'm always open to being convinced otherwise).

If I were asked today what to upgrade in the Mainline, I would strongly urge building the stock kit first and then considering the Auricaps if you like the stock sound. If you yearn for something warmer or lusher - more tubey perhaps - then the Mundorf Supremes might be a better choice (write up of Mundorf Supreme Silver / Gold / Oil coming to the S.E.X. review soon), but I definitely think there's huge value in hearing the stock kit first so you know which way to tweak (or not tweak) the sound if you choose to upgrade the output (technically parafeed coupling) caps. Unless something changes in the coming days / weeks in which case I'll be back to edit this post, I wouldn't really recommend changing the regulator caps. Keep the build neat and simple.

This should be a great thread! I look forward to your comparisons and hope to share some of my own as well in the not too distant future. My mainline build is coming along - currently finishing off the last half dozen or so resistors in the coarse attenuator.

11-18-13 Update - Initial Impressions of Mainline:

Having finished my mainline over the weekend and sat back with my hd800's and listened through a sampling of my favorite tracks this evening (from Oscar Peterson and Jazz at the Pawnshop k2hd to Emma Kirkby and the Choir of St. Martins, and from Alison Krauss to 2Pac) - I have to seriously applaud the team at bottlehead on their new flagship. This amp is technically excellent and thoroughly enjoyable with every genre you can throw at it, and that is most definitely not a statement that I would make about many widely recommended "TOTL" headphone amps.

To provide something a bit more concrete - here are a few preliminary impressions of what I'm hearing in reference to Pierre Fournier's brilliant performance of Bach's Cello Suites. I've probably listened to this album a hundred times on various systems and can't ever recall hearing it sound this good. Transients are simply effortless - so fast and clean. Timbre is rich and lifelike, and the audible micro-detail and texture as Fournier digs deep into the strings on certain notes are absolutely amazing. More generally, on albums across the board, bass presentation is faultless - deep, impactful, and incredibly tight. Possibly the best I've heard from any tube amp? Trebles are detailed and nicely extended, but never strident. For example, I tend to prefer my crack with the anax mod in place. But to my ears, the pairing with the mainline doesn't need it and hd800's have more air without.

From a comparative standpoint, I will always love my crack (an incredible value which will soon take up residence in my office) - but for anyone who has been sitting on the fence awaiting comparisons, be assured that, at least to my ears, the mainline absolutely takes it to the next level.

The mainline retains what I have come to think of as bottlehead's house signature (fast, dynamic and engaging - making your toes tap - with a hint of tube fullness and warmth - the tubes, in a sense, filling in for the room effects you don't get with headphones), but it is just incredibly clean and refined, particularly in its excellent bass response and handling of transients, compared to my modded crack. This is no doubt the result of its ultra highly regulated single gain stage topology and excellent custom output transformers. Based on my recollections from CanJam - I would analogize that mainline is as fast, detailed and dynamic as a Zana Deux (leaving slower "tubier" flagships like the Woo 5 in the dust), but with a slightly sweeter top end. Trumpets still scream when the source material calls for it, but I can't imagine this amp will ever be characterized as even slightly harsh or piercing - which criticisms are occasionally aimed at the Zana. (I imagine this may mean that mainline has really low high order distortion - sort of the antithesis of the disproportionate glare folks complain of in SS amps with lots of global feedback?) In any event, these positive qualities essentially mean that the mainline is a perfect match for hd800's to my ears. Even when listening to very simple music (i.e. the Wailin Jenny's - the Parting Glass), the incredibly clean separation between voices is an obvious improvement over other amps with which I am familiar and makes vocal harmonies that much more enjoyable. The same obviously applies to fast complex music as well, where the mainline's control and handling of transients is consistently impressive. The more I listen, the more enamored I become.

Hope more of you will have a chance to hear this amp in the near future. If possible, I will bring mine to the So. Cal. meet in March.

This should be a great thread! I look forward to your comparisons and hope to share some of my own as well in the not too distant future. My mainline build is coming along - currently finishing off the last half dozen or so resistors in the coarse attenuator.

[In any event, I'll reserve this spot for a bit of crack v. mainline compare and contrast.]

FWIW I rewrote some recommendations regarding the choice of amp for our soon-to-be-overhauled web site. It was old enough that it discussed our retired Smack kit, so it now includes Mainline instead. I won't post it here because that could easily be construed as advertising. But It should be up on the new site in the next month or two.

FWIW I rewrote some recommendations regarding the choice of amp for our soon-to-be-overhauled web site. It was old enough that it discussed our retired Smack kit, so it now includes Mainline instead. I won't post it here because that could easily be construed as advertising. But It should be up on the new site in the next month or two.

Brilliant. Thanks Doc. Any chance you could paste that content into a PM or email for me? Completing the SEX has me excited about the potential from the Mainline so I'd be interested to read your recommendations so I can relate it to my current tastes and equipment.

FWIW I rewrote some recommendations regarding the choice of amp for our soon-to-be-overhauled web site. It was old enough that it discussed our retired Smack kit, so it now includes Mainline instead. I won't post it here because that could easily be construed as advertising. But It should be up on the new site in the next month or two.

Doc,

Please put the current comparison into the Bottlehead archive / Community section, so it is still accessible to those of us who own Smacks.